Transaction ID wraparound prevents new transactions

When database reaches fewer than 3 million transactions before wraparound (2 billion transaction limit), PostgreSQL refuses to assign new XIDs to prevent catastrophic data loss. System emits warnings at 40 million transactions from wraparound.

Resolve old prepared transactions via pg_prepared_xacts. End long-running transactions found in pg_stat_activity with large age(backend_xid) or age(backend_xmin). Drop old replication slots from pg_stat_replication with large age(xmin) or age(catalog_xmin). Execute database-wide VACUUM (not VACUUM FULL). Do not stop postmaster or enter single-user mode.